Some inkjet printing systems or printers that treat the image receiving surface with surface preparation materials include a cleaning device to remove certain materials from an image surface without removing all of the surface preparation material for the next printing cycle. Surface preparation material is any substance applied to the image receiving surface to enable an ink image to be formed on the surface and to facilitate the transfer of the ink image from the surface to media. Examples of a surface preparation material or a blanket coating include, but are not limited to, a skin coating, a fluid coating, a combination thereof, or the like. In some previously known systems, a blade cleaner is used to remove materials from the image surface. The materials removed from an image surface to replenish the ability of the image surface to form quality images include ink, surface preparation substances, media debris, and the like. Blade cleaners are effective because they can provide higher pressures on the imaging surface, but these pressures can result in a shorter life of the image forming surface and the blade cleaner. Additionally, blade cleaners need a higher blade load required to clean the image surface. The blade cleaners also have a poor reliability because they have a single cleaning edge that slides across a high friction elastomer blanket surface.
In some previously known system, a web cleaner is used to remove materials from the image surface. However, web cleaners have a high consumable cost of web materials and cost of disposal of the webs. While the fiber edges on the web can provide a better redundant cleaning with respect to the blade, the thinness of the web provides little volume for storing the detached ink as it is transported out of the cleaning nip. As such, a web cleaner must be translated through the nip at a rate to transport cleaned ink out of the nip faster than the rate at which the ink enters the nip. Additionally, the web cleaners also have a limited cleaning capacity. The limited ink capacity of webs makes them impractical in high ink density situations.
To address the issues related to blade and web cleaners, some previously known aqueous ink printing systems have used a foam roller that rotates against the movement of the image receiving surface to scruff and carry material away from the surface. In aqueous ink printing systems, the image receiving surface that is cleaned by the foam roller is a blanket of material wrapped around an endless support surface, such as a rotating drum or belt. To enhance the surface properties of the blanket so ink adheres to it during image formation and then releases the ink image during transfer to media, the blanket is treated with a surface preparation material that forms a skin on the blanket surface. This surface preparation material is applied to the surface of the blanket after the ink image has been transferred to media and the blanket surface has been cleaned of the skin and residual ink from the previous imaging cycle. Ideally, the pressure of the foam roller should split and remove the ink layer while only hydrating the skin layer so it can be replenished. If the pressure applied to the blanket by the foam roller is too high, however, the thin skin layer under the ink layer also splits. This splitting of the skin layer enables some of the loosened ink to contact the blanket surface, which has an affinity for the ink. Consequently, the ink adheres to the blanket surface and is harder to remove than ink on the skin preparation material. Thus, the cleaning of the blanket is adversely impacted and image quality can be affected in subsequent imaging cycles.
In certain previously known aqueous ink printing systems, the ink is dried to a semi-wet consistency to enable the transfer of the ink image onto media before the imaging surface is cleaned by the cleaning device. In most cases, the semi-wet ink is easier to clean since the density of the ink is small. However in certain cases, the ink is over-dried. Over-dried ink can occur regularly in machine operation due to machine faults. For example, faults such as media handling faults, control faults and other situations can result in the machine shutting down during the printing operation. The processing of these faults can leave the ink image under the dryers longer than desired. The extra drying can make the dried ink harder to clean. Over drying can also reduce efficiency of the ink image transfer to the media causing a larger amount of the harder-to-clean ink to be introduced to the cleaning device in the printer system. To compensate for the occurrence of these situations, a blade cleaner may be employed since the blade cleaner can apply the higher pressures required to remove dried ink with the attendant risks noted previously.
FIG. 4 is a graph illustrating the effect that drying the aqueous ink on the image forming surface to various degrees has on the cleaning performance. Particularly, the effect of drying the ink on a blanket surface was tested on the load of a blade cleaner. Line 404 in FIG. 4 represents an over-dried ink, line 408 represents a semi-wet ink, line 412 represents undried ink, and line 416 represents water. The vertical axis of the graph in FIG. 4 represents the blade load in g/cm and the horizontal axis of the graph represents the blade working angle in degrees. In previously known aqueous ink printing systems, machine operators resort to isopropyl alcohol soaked rags to remove the over dried ink rather than using water alone. However, as seen in the graph in FIG. 4, the effort required to remove ink rises significantly when over dried ink 404 is cleaned from a blanket by hand. As such, improvements in inkjet printers that enable cleaning of the imaging surface are desirable.